Reduce redundant CI matrix jobs

[sapunyangkut]

Summary

• keep the tox packaging and documentation test path on Ubuntu
• retain the three-platform pixi matrix for OS coverage
• run the downstream pixi install smoke test on Ubuntu and remove its now-unneeded platform branch
• remove the redundant import check from the pixi job, since the test step imports the package

This reduces the workflow from 10 runner jobs to 6 while preserving each distinct test path.

Validation

• actionlint 1.7.12 .github/workflows/ci_workflows.yml
• parsed the workflow and checked the expected runner/matrix structure with PyYAML

Closes #227

[Copilot]

Pull request overview

This PR reduces redundant GitHub Actions CI runner jobs by consolidating OS matrices: keeping the tox-based packaging+docs test path on Ubuntu while preserving 3-OS coverage via the pixi job, and slimming the downstream pixi smoke test to Ubuntu-only.

Changes:

• Make the tests (tox) job run on ubuntu-latest only instead of a 3-OS matrix.
• Remove the redundant pixi “Verify install” step (tests already import the package).
• Make pixi-downstream run on ubuntu-latest only and remove the Windows/Unix branching for adding the local package.

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